A rechargeable secondary battery or battery pack is used more widely in mobile phones, notebook computers, electric tools, electric cars (including hybrid, plug-in hybrid and all-electric cars), backup power systems, and other energy storage systems. In these applications, operation of a battery is generally not within an entire range of an SOC; therefore, it is important to accurately understand a state of charge (state of charge, SOC) of a battery. However, it is usually inaccurate to determine the SOC of a battery by detecting a voltage of the battery. For example, for a lithium-ion battery, a platform voltage exists in a range with an approximately 50% SOC as a center, and a voltage of the battery varies slightly with the SOC in the range of the platform voltage. In addition, the voltage of a battery cannot reflect an electric potential at an electrode (that is, the SOC of the electrode). In an operation process of a battery, when an electrode potential shifts to a certain direction, the battery safety becomes lower but the battery voltage does not change or changes slightly. Therefore, the potentials of electrodes need to be detected to accurately determine the SOC of a battery.
In order to detect the electrode potential at an anode and a cathode of a battery, a third electrode, namely a reference electrode, must be introduced, so that the third electrode as well as the anode and the cathode of the battery together form an electrochemical three-electrode battery system. In the electrochemical three-electrode system, a potential at the reference electrode is required to be always constant. Otherwise, the reference function does not take effect. A specification and location of the reference electrode are important for measuring the electrode potential. If a large polarized electric potential exists between the location of the reference electrode and a measured electrode, accuracy of the electrode potential measured by using the reference electrode is affected by polarization. Therefore, how to introduce and locate a reference electrode are key technical issues. FIG. 1 is a schematic diagram of a battery structure in the prior art, where a reference electrode 1 is located at a cathode end and close to a cathode cover, and a leading-out terminal 2 of the battery is located near a cathode 3. A material of the reference electrode 1 is lithium metal, a lithium alloy, another lithium transition metallic oxide such as a lithium-titanate oxide, or the like. In addition, a porous material is used to encapsulate the reference electrode 1, so as to implement electrical insulation from the anode and cathode. A battery structure designed according to this technical solution has a low cost but does not completely solve a problem that polarization affects the electrode potential. A reason is that: although the reference electrode 1 is close to the cathode cover, there is still a certain distance from the cathode and the reference electrode 1 is led out through an independent terminal 2, and a measured electric potential is not the electric potential at either electrode of the battery. As a result, the electric potential at neither the anode nor the cathode can be measured in an accurate and reliable manner in this solution. In addition, the solution adds an extra electrode leading-out terminal, which results in complication of a manufacturing technique and an increase of battery leakage risk.
Considering the safety of a battery, temperature and internal pressure of the battery during the operation process are also factors that technicians often concern. At present, a method commonly used in the lithium-ion battery field is that a thermal element (for example, a positive temperature coefficient resistor, PTC) is added to an external battery management system to prevent thermal runaway caused by an excessively large current in a circuit, or an explosion-proof valve is disposed on a shell of the battery to prevent an explosion caused by an excessively high internal pressure.